And there was no evidence of what has been called risk compensation -- increasing sexually dangerous behavior as result of taking PrEP, he said.

At least where it is offered free of charge and delivered by a trained clinician, Grant said, PrEP will work outside of a clinical trial.

The original iPrEX trial tested the idea that taking a single pill containing two antiretroviral medications -- tenofovir and emtricitabine (Truvada) -- would help prevent catching HIV among people at high risk for the disease.

The randomized blinded trial found that the pill-a-day strategy reduced the risk of HIV by 42% among a cohort of men and transgendered women who have sex with men.

The iPrEX OLE follow-on study -- the name, with a Spanish pronunciation of the last word, was "intended to be playful," Grant told reporters -- included 1,603 HIV-negative people from six countries, all considered at high risk for HIV.

Many, but not all, had been in the original trial, while others had participated in other PrEP studies.

Grant said 76% of the participants decided they wanted to take PrEP, so "uptake was high." Other participants remained in the cohort but did not take the medication.

Interestingly, he said, people at higher risk for HIV -- those who practiced anal sex without a condom, for instance -- were more likely to be involved in the trial, more likely to start PrEP, and more likely to adhere to the medication.

"People who needed PrEP the most used it the most," he said.

That was a contrast to skeptics' warnings that the only people who would follow the regimen would be those who already were taking other precautions on their own, Grant told reporters in a briefing before his late-breaker presentation.

The rate of HIV infection among the participants who started PrEP was 1.83 cases per 100 person-years, Grant told MedPage Today -- 49% lower than the 2.6 cases per 100 person-years among non-PrEP participants, after adjustment for baseline differences in sexual behavior.

Grant noted that when adherence was good -- four or more tablets a week -- there were no cases of HIV infection. On the other hand, over the 72 weeks of follow-up, participants reached that level only 33% of the time.

Adherence was measured by sampling dried blood spots, he reported. When sampling indicated no drug in the blood, HIV incidence was 4.7 cases per 100 person-years.

That fell to 2.2 cases per 100 person-years among people taking some but less than two tablets a week and to 0.6 cases per 100 person-years if people took two or three pills a week, he said.

The findings suggest that "adherence has to be good but it doesn't have to be perfect" to prevent HIV.

The study is "real-life proof" that PrEP can work, commented Stefano Vella, MD, of the Istituto Superiore di Sanita in Rome, and a former president of the International AIDS Society.

It's especially important, he told reporters, that the study answers the question of whether adherence to the medication has to be 100%. "These are very, very important data," he said.

The study had support from the NIH and Gilead Sciences supplied the study drug. Grant made no disclosures.

Vella made no disclosures.

Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco

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